The field of the invention is cholera vaccines.
Cholera is a severe and sometimes lethal diarrheal disease caused by the Gram-negative bacterium Vibrio cholerae. Historically only the O1 serogroup of V. cholerae has been associated with epidemic cholera. However, in early 1993 in India and Bangladesh, a major cholera epidemic was caused by a novel non-O1 serogroup of V. cholerae named V. cholerae O139. Strains belonging to this newly emerged V. cholerae serogroup replaced the endemic El Tor O1 strains of V. cholerae to become the principal clinical and environmental isolate of V. cholerae on the Indian subcontinent (Cholera Working Group, 1993, supra).
The initial microbiologic characterization of V. cholerae O139 revealed that this serogroup was closely related to the El Tor biotype of V. cholerae O1. The shared properties of V. cholerae O139 and El Tor O1 strains include (1) the agglutination of chicken red blood cells; (2) resistance to polymyxin B (Cholera Working Group, 1993, Lancet 342:387-390, 1993); (3) in vitro growth conditions for the expression of virulence factors (Waldor et al., Infect. Immun. 62:72-78, 1994); (4) identical sized restriction fragments for genes which have known polymorphisms (Calia et al., Infect. Immun. 62:1504-1506, 1994; Waldor et al., supra); (5) identical electrophoretic types by multilocus enzyme electrophoresis analysis (Popovic et al., J. Infect. Dis. 171:122-127, 1995); (6) tandem duplications of the CTX genetic element (Waldor et al., J. Infect. Dis. 170:278-283, 1994); and (7) identical chromosomal location of the CTX genetic element (Waldor et al., 1994, supra). These findings support the hypothesis that V. cholerae O139 is a derivative of an El Tor O1 strain of V. cholerae. DNA sequence analysis of tcpA, which encodes the major subunit of the toxin co-regulated pilus TCP in El Tor O1, classical O1, and O139 strains has given strong support to this hypothesis (Iredell et al., FEMS Microbiol. Lett. 121:47-54, 1994; Rhine et al., Mol. Microbiol. 13:1013-1020, 1994). While there is approximately 30% difference in the sequence of tcpA between classical and El Tor O1 strains, the O139 and El Tor tcpA sequences were identical (Rhine et al., supra). Recent analyses of the sequences of the gene encoding aspartate-semialdehyde dehydrogenase in various strains of V. cholerae also support a closer genetic relationship between O139 strains with El Tor O1 strains rather than with classical O1 strains.
While V. cholerae O139 shares many characteristics with the El Tor O1 strains that were endemic on the Indian subcontinent at the time V. cholerae O139 arose, O139 strains had two principal features which distinguished them from the El Tor O1 strains (Nair et al., J. Clin. Microbiol. 32:2775-2779, 1994). These features were the novel O139 serogroup antigen and a distinct set of antibiotic resistances. The gram-negative Bacteroides obligate anaerobe conjugative transposons range in size from 65 kb to 150 kb, generally encode resistance to tetracycline, and sometimes contain genes encoding resistance to erythromycin and clindamycin as well (Salyers et al., J. Bacteriol. 177:5727-5731, 1995). The conjugative transposons described in Gram-positive bacteria have similar properties but at least in the case of the Tn916-Tn1545 family, they do not exhibit a high degree of insertion site-specificity and excise from the donor strain independent of recA (Clewell et al., Trends Microbiol. 3:229-236, 1995). These Gram-positive conjugative transposons generally encode resistance to tetracycline as well other antibiotics (Clewell et al., supra). Previous studies have demonstrated that trimethoprim resistance genes in V. cholerae are plasmid- or transposon-encoded (Gerbaud et al., Ann. Inst. Pasteur/Microbiol. 136B:265-273, 1985).
For individual safety and public health reasons, a safe cholera vaccine should be sensitive to as many commonly used antibodies as possible.